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Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

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484                                ALASKAN GLACIER STUDIES
posits are lacking in Prince William Sound, then the recession of the expanded glaciers of Prince William Sound was different in nature from that of Yakutat Bay. It is possible that when the ice supply ceased to maintain the expanded piedmont glacier near its outer limit the ice margin was mainly in the waters of the sound and that the marginal deposits were made beneath the sea. This is a problem whose solution must depend upon further work, and it is possible that such work will discover marginal deposits in parts of the region which we did not visit.
Readvance of Glaciers. In Yakutat Bay there is clear evidence of a recent, but brief, great advance of the glaciers, during which they overrode but failed to completely remove a series of gravels of earlier date. Before this advance the glaciers had been farther back than now and forests spread above where the present glacier termini stand, as well as throughout the fiord where the overridden gravels lie. The recent history has been one of retreat and the forest has not yet advanced up to the glacier fronts, although formerly extending beyond them.
In Prince William Sound the glacier history has been notably different. Some of the glaciers are and have for some time been in recession, but even here there is no such extensive semi-barren zone between the glacier fronts and the forest as in Yakutat Bay. It is inferred, therefore, that where glaciers are retreating in the Prince William Sound region the recession has not been as rapid or as extensive as in Yakutat Bay. We have found no evidence in our study of Prince William Sound of an episode of later advance and retreat similar to that of Yakutat Bay, although we find a suggestion of one near by on the Copper River. Thus we conclude that, although in the period of greatest expansion of glaciers there was a probable synchronism of advance and retreat, since that time the two regions have had a quite differenthistory. The glaciers of YakutatBay readvanced notably, but those of Prince William Sound apparently did not; the recent history of the Yakutat Bay glaciers has been one of rapid retreat, that of Prince William Sound has not, and, in at least one case, while general retreat was in progress in Yakutat Bay notable advance was in progress in Columbia Glacier and 14 other ice tongues of Prince William Sound. Ultimately these facts of difference in behavior of glaciers in different parts of the same coastal mountain region will doubtless have importance in interpretation of the causes of the difference; at present, with the limited range of our facts, we cannot present a verified interpretation. Whether it is due to local climatic variations, or to difference in time of response to more general climatic variations, or to influence of earthquake shaking, we cannot now determine.
Piedmont Glaciers and Ablation Moraines. Owing to the fact that large glaciers have extended with abundant supply from lofty mountains out upon lower, fairly-level land at their base, many of the glaciers of St. Elias-Pairweather mountains have spread out at the mountain base in piedmont bulbs and ice plateaus. This condition is characteristic of such glaciers in their lower ends in the Yakutat Bay region and its neighborhood to the northwest and southeast. It is entirely absent from the Prince William Sound regions, because one of the factors which give rise to it is absent, namely the termination of glaciers on lower, level land. Some of the glaciers end in the fiords, and these cannot, of course, assume the piedmont bulb form because expansion is checked by iceberg discharge due to the attack of sea water. Others, that end on the land, are confined in narrow valleys, and here too lateral expansion is prohibited. It is possible that some glaciers from side valleys entering larger main valleys may in this region expand into